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Tue Aug 9 15:58:14 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-August/145807.html

On 09.08.16 00:59, Chris Angelico wrote:
> class TooSimpleContextManager:
>     """Now this time you've gone too far."""
>     def __init__(self, func):
>         self.func = func
>     def __call__(self):
>         self.gen = self.func()
>         return self
>     def __enter__(self):
>         next(self.gen)
>     def __exit__(self, type, value, traceback):
>         try: next(self.gen)
>         except StopIteration: pass
[...]
 > My numbers are:
 >
 > 0.320 secs
 > 1.354 secs
 > slowdown: -4.23x
 > 0.899 secs
 > slowdown: -2.81x
 > 0.831 secs
 > slowdown: -2.60x
 > 0.868 secs
 > slowdown: -2.71x

I have got a slowdown 1.74x with TooSimpleContextManager with following 
implementation of __enter__ and __exit__:

     def __enter__(self):
         for res in self.gen:
             return res
         raise RuntimeError("generator didn't yield")
     def __exit__(self, type, value, traceback):
         for _ in self.gen:
             raise RuntimeError("generator didn't stop")

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Showing content from https://mail.python.org/pipermail/python-dev/2016-August/145807.html below:

[Python-Dev] Rewrite @contextlib.contextmanager in C